hey noob, i know ya conundrum, i had the same question but an old mechanic told me that it comes from the German words 'differential' and 'access'.

You see a lot of the old workshops in the fatherland couldn't afford the fancy hoists you see everywhere today. so the clever little chaps at BMW thought 'insert light bulb' hey why not a removable panel to access the diff. so differential and access became, diff - user.